This invention relates to novel antifouling membranes produced by integrating fillers exhibiting visible light photo-catalytic activity.
Polymeric membranes have been widely used for desalination and water softening, membrane bioreactors for wastewater treatment, biomedical and pharmaceutical materials separation, and other chemical engineering applications. However, undesirable fouling of membranes decreases their permeability, reduces their permeate quality, and increases the energy costs of a separation process in which the membranes are used.
The pioneering paper of Fujishima and Honda [1] on water splitting opened the way to significant research and studies of titanium dioxide (TiO2). Now, TiO2 is widely used for water splitting, water treatment, air purification and self-cleaning of surfaces. This is because of its unique photo-catalytic properties, stability, commercial availability, and ease of preparation [2, 3]. With the increasing awareness of environmental issues, TiO2 is considered an ideal choice as a catalyst for water treatment due to its high oxidation power, photo-induced hydrophilicity, long-term photo-stability, high transparency in the visible range of light, good thermal and chemical stability, and non-toxicity [4, 5]. However, technical barriers have hindered its commercialization. These include the high level of energy input (UV light) required to implement photo-degradation, and post-recovery of catalyst particles after water treatment [5].
Because of its relatively large band gap of 3.2 eV, TiO2 (anatase) can only be activated by UV light having a wavelength shorter than 380 nm [6]. Accordingly, TiO2 can only take advantage of 3-4% of the solar energy reaching the earth [7]. Many attempts have been made to prepare a TiO2 photo-catalyst which can be activated under visible light with reasonable efficiency. These attempts include dye sensitization [8, 9], noble metal deposition [10, 11], and metal or non-metal doping [6, 12, 13]. Since the discovery by Asahi et al. [14] of N-doped TiO2 with a visible light absorption, a great amount of attention has been given to modifying the electronic band gap of TiO2 using a doping method. Various elements including Nitrogen (N) [15, 16], Fluorine (F) [17], Sulfur (S) [18, 19], Iron (Fe) [20], Cobalt (Co) and Chromium (Cr) [21], Copper (Cu) [22], and Manganese (Mn) [23] have been studied in the attempt to enhance the photocatalytic performance of TiO2 in the visible light region. Compared to other nonmetal elemental doping, N-doped TiO2 materials exhibit a significant photocatalytic activity under visible light irradiation. This is probably because N 2p states mix with O 2p states due to the fact that the respective energy levels are very close to each other [14, 24].
Many researchers have coupled TiO2 photocatalysis with membrane separation to solve the problem of post-recovery. For example, Xi and Geissen [25] reported the separation of TiO2 from water by cross-flow microfiltration (MF) within wastewater treatment by photocatalysis using slurry reactor systems. In their study, separation efficiency was strongly affected by cross-flow velocity, transmembrane pressure (TMP), feed concentration, pH of the suspension, and ionic strength. Le-Clech et al. [26] reported that a hybrid photocatalysis-Poly(vinylene fluoride); i.e., PVDF, MF membrane process was effective as a polishing treatment of surface water containing low concentrations of natural organic matters. During the process, the membrane would totally reject TiO2 slurry particles and separate them from the treated water.
Due to the hydrophilic properties of TiO2, incorporation of this type of nano-particles (NPs) into the membrane structure would enhance the composite material's affinity to the water and hence membrane water permeation and fouling resistance [27, 28].
So, combining TiO2's unique properties with membrane technology through integrating TiO2 with the membrane provides an opportunity for advanced water treatment. This integration not only keeps the characteristics and capacity provided by these two technologies, but also produces certain synergistic effects which overcome some the drawbacks of the single technology [29]. First, pollutants could be oxidized by the photocatalysis, while the membranes demonstrate the capability not only to retain the photocatalyst, but also to partially reject organic species by controlling their residence time in the reacting system. That is, the membrane works as a selective barrier for targeted molecules. Therefore, this technology can enhance photocatalytic efficiency and achieve excellent effluent quality. Second, the integrating technology can solve or mitigate the problem of flux decline associated with membrane fouling [25]. Because TiO2 particles on the membrane surface not only increase membrane hydrophilicity, but also create a photocatalytic property, formation of cake layer and blocking of membrane pores are prevented which can extend membrane life.
There are two main approaches for fabricating TiO2 nanocomposite membranes: first, depositing NPs onto membrane surface, and, second, blending the NPs into the membrane.
In the depositing approach, TiO2 NPs are coated or grafted onto the membrane's outer surface. For example, Kuak et al. [30] and Kim et al. [31] prepared one kind of hybrid composite membrane by self-assembly of TiO2 NPs through interaction with the COOH functional group of an aromatic polyamide thin-film layer. The membrane possessed an excellent anti-bacterial effect on E. coli and good antifouling properties under UV light irradiation. Bae and Tak [32] immobilized the TiO2 NPs on the membrane's surface using a dipping method so to increase the surface hydrophilicity for filtration of a mixed liquor from a membrane bioreactor (MBR). Test results showed that membrane fouling was considerably mitigated though the flux still declined to some extent.
In the blending approach, TiO2 NPs are dispersed in a casting solution and then membranes are cast by a phase separation method widely used for the preparation of polymeric membranes. For example, Bae and Tak [33] entrapped TiO2 NPs in a polymeric membrane to mitigate fouling during active sludge filtration. Wu et al. [34] prepared Polyethersulfone (PES)/TiO2 composite membranes which showed enhanced hydrophilicity, thermal stability, mechanical strength and antifouling ability. Thermogravimetric analysis (TGA) and mechanical strength analysis also indicated good compatibility between polymers and TiO2 NPs. To avoid agglomerations and also to improve the stability of particles in the casting solution, Razmjou et al. [35] modified Degussa P25 TiO2 NPs by a combination of chemical processes and mechanical methods for fabricating ultrafiltration (UF) membranes. The incorporation of modified NPs into PES UF membranes showed a significant improvement in fouling resistance with an increase in hydrophilicity being the most likely reason for the improvements in antifouling performance. Compared to the depositing approach, the blending approach is simpler since the particles are added to the casting solution. Furthermore, the coating of membranes can result in significant undesirable changes in membrane permeability due to pore narrowing or plugging. Potential delamination of the coating layer is also a concern [35].